feat: support ramping-in of volume

[fivecar]

This is an RFC for supporting the fading in of audio when you play(). The idea would be to support AVPlayerWrapper.rampSecs, which, if set to non-zero, fades audio in every time you play. The broader proposal would be to expose this through RNTP at some point so that people can implement players where audio ramps in whenever Play is hit.

Eager to hear feedback on this. Alternatives to this include having clients just setTimeout themselves, manually calling setVolume a bunch of times with incrementally-increasing values, but: a) a native implementation ramps volume much more smoothly, and b) this makes it effortless to create players that always ramp sound in.